Electrodeposition of copper



Patented Oct. 5, 1954 STA ATENT OFFICE ELECTRODEPOSITION OF COPPER No Drawing. Application October 21, 1952, Serial No. 316,085

9 Claims. i

This invention relates to copper cyanide electroplating electrolytes containing lithium as an addition agent, and to the process for electroplating copper therefrom.

It is highly desirable in the electroplating industry to be able to increase the rate of electrodeposition of metal while maintaining quality at a predetermined level. Ordinarily, as the plating current density is increased above a certain optimum value, the amount of metal deposited per unit will increase, but the deposit tends to be rougher, burned, non-uniform and otherwise inferior. Therefore, the amount of plating that may be produced from a given plating tank is limited by the electrical current that may be passed therethrough. Any expedient that will enable more current to be passed through the electrolyte and thus more copper plated on a member without sacrificing quality, represents a desirable and wanted gain.

In accordance with the present invention, it has been discovered that aqueous copper cyanide electroplating electrolytes may be greatly improved by adding thereto small critical amounts of lithium, whereby the electrodeposition of copper therefrom, without sacrifice of quality, may be very substantially increased. In some instances, there is an improvement in quality attended with a higher rate of plating.

The beneficial results of lithium additions to aqueous copper cyanide electroplating electrolytes are particularly pronounced when periodic reverse current is applied thereto.

The object of this invention is to provide for the electrodeposition of copper from aqueous copper cyanide electrolytes containing small critical amounts of lithium.

A further object of this invention is to provide an aqueous cyanide electroplating electrolyte containing small critical amounts of lithium therein as an addition agent to enable satisfactory plating at a higher rate than would be possible in the absence of the lithium.

Another object of the invention is to provide for electroplating copper by means of periodic reverse current from an aqueous cyanide copper electroplating electrolyte containing lithium a an addition agent.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

I have discovered that electrodeposition of copper from an aqueous copper cyanide electroplating electrolyte may be greatly benefitted by adding to the electrolyte from 0.01 to 0.4 ounce of lithium per gallon. The presence of lithium in these proportions enables the electrodeposition of copper from the electrolyte at a rate of about from 20% to 25% greater than would be otherwise possible from the same electrolyte, with the copper deposits being of substantia ly equal quality.

Furthermore, the lithium additions may be added to copper cyanide electrolytes containing either zinc in the proportions from 0.004 to 0.57 ounce per gallon, or from 0.1 to 10 ounces per gallon of thiocyanate (CNS-) in the electrolyte, or both. The combination of lithium, zinc and thiocyanate in an aqueous copper cyanide electrolyte has enabled the plating of copper in commercial installations at the highest rate known heretofore for given members.

The electrolytes from which copper is to be electroplated may be potassium cyanide electrolytes, sodium cyanide electrolytes, mixed potassium and sodium cyanide electrolytes, and Rochelle copper cyanide electrolytes; and when the lithium, zinc and thiocyanate are introduced therein in the proportions herein set forth, the electrodeposition of copper by periodic reverse current is markedly benefitted.

A copper cyanide electroplating electrolyte may be prepared with from 2 to 15 ounces per gallon of dissolved copper, an alkali metal cyanide in an amount to complex the copper in solution, that is, to produce the more soluble alkali metal-copper cyanide double salt, and in addition to provide for 0.5 to 5 ounces per gallon of free alkali metal cyanide, from 2 to 12 ounces per gallon of alkali metal hydroxide, and up to 20 ounces per gallon of alkali metal carbonate. The cyanide solutions are usually maintained at a pH of from 12 to 13.5 and slightly higher. The electrolyte may contain other components commonly present in cyanide copper electroplating electrolytes such, for example, as 3 to 8 ounces per gallon of Rochelle salts (potassium sodium tartrate).

The lithium may be introduced into the electrolyte in the form of any one of a number of lithium salts that are soluble in the plating solution. Thus, the lithium may be introduced as lithium hydroxide, lithium nitrate, lithium carbonate, lithium citrate and lithium thiocyanate. By using the lithium thiocyanate, both lithium and thiocyanate are added simultaneously to the electrolyte. The optimum benefits are obtained when lithium is present in the proportions of from 0.04 ounce to 0.1 ounce per gallon of the cyanide electrolyte.

In the case that zinc is also to be added to the electrolyte, it may be added as zinc oxide, zinc cyanide, or zinc thiocyanate, or other water soluble salts. In some cases, the zinc may be added by introducing a zinc anode into the copper cyanide plating bath and passing a deplating current to the zinc anode, whereby to dissolve zinc in the electro yte.

In some cases, a composite mixture may be prepared for introducing lithium, zinc and thiocyanate into the electrolyte. In general, it is desirable to include in such composite mixtures a major proportion of sodium cyanide which tends to promote solubility of the salts in a copper cyanide electrolyte. The following examples are typical of such composite addition agent mixture in which all parts are by weight:

Parts ((1) Zinc oxide '7 Lithium hydroxide monohydrate 5 Potassium thiocyanate Sodium cyanide 78 (1)) Sodium cyanide 45 Sodium thiocyanate 5 Lithium hydroxide monohydrate 50 (c) Lithium thiocyanate 40 Zinc oxide 10 Sodium cyanide 50 One important operating advantage derived by the use of combined lithium, zinc and thiocyanate additions in copper cyanide electrolytes is that the resulting electrolyte can be treated with activated carbon and filtered with very little loss of either lithium, zinc or thiocyanate. Metal thiocyanate salts are considered to be inorganic compounds. The commonly used organic addition agents, which are usually quite expensive, are removed rather completely from copper cyanide electrolytes when subjected to the same activated carbon treatment. Therefore, the expense of replenishment of removed addition agent is avoided by the present invention. Because of this fact, the electrolytes of the present invention can be economically treated with activated carbon at any time or even continually to remove impurities and thereby are maintainable in most efficient plating condition.

Aqueous copper cyanide electroplating electrolytes, containing lithium as an addition agent, can be employed with most satisfactory results with periodic reverse current. As disclosed in my Patent 2,451,341 and in a copending application of which I am a co-inventor, Serial No. 125,798, periodic reverse current plating involves a cycle of plating followed by deplating and comprises the passing of a plating electrical current through the member being plated while it is in contact With an electroplating electrolyte for a period of time of the order of from /5 second to 300 seconds and then reversing the fiow of electrical current, usually for a shorter period of time and at a current density so that the coulombs of deplating current comprise from 10% to 90% of the coulombs applied during the plating period. The plating portion of the periodic reverse current cycle deposits an increment of copper on the member being plated, while the succeeding deplating portion of each cycle deplates a portion of the previously plated copper proportional to the coulombs of the deplating current. The net result of a complete cycle of plating and deplating current results in a net increment of sound, homogeneous and uniform copper on the member. Repetition of the plating and deplating portions of the cycle builds up a progressively smoother and more uniform electrodeposit on the member until the desired thickness of copper has been secured. Ordinarily, the smoothest and most uniformly distributed deposits of copper result from the use of cycles in which the deplating current is of such a magnitude and applied for a period of time that the coulombs exceed 30% of the plating coulombs. Also, it is desirable particularly With cycles in which the plating current exceeds 40 seconds that the deplating current density be less than the plating current density and preferably not exceeding of the plating current density. It will be understood by reference to the patent and patent application referred to herein that the cycles may be varied within a considerable latitude.

The following examples illustrate the practice of the present invention.

Example I An aqueous electrolyte having the following composition was prepared:

Ounces per gallon Copper 11.6 Sodium hydroxide 4.4 Free potassium cyanide 1.9 Potassium thiocyanate 1.8

Into this electrolyte, wire of a diameter of approximately 0.06 inch was passed while mounted on a revolving reel. The reel revolved at 50 revolutions per minute. To the wire there was applied a periodic reverse current having a cycle having a 15 second plating period and a 3 second deplating period, the current density being equal in both plating and deplating periods. Tests showed that to product good copper plating on the wire, the maximum allowable plating current density was amperes per square foot. Any increase in plating current density above 100 square feet resulted in building up of nodules and the burning of the copper on the wire.

To the electrolyte of this example, there was then added 0.25 ounce of lithium hydroxide monohydrate (LiOH'H2O) per gallon, which introduced approximately 0.04 ounce per gallon of lithium into the electrolyte. The maximum current density that could be applied to secure a copper deposit equal in quality to that previously obtained had increased to amperes per square foot.

In another test, the electrolyte of this example was modified by adding thereto lithium hydroxide monohydrate to provide a total of 0.08 ounce of lithium per gallon. A plating current of amperes per square foot could be applied to the wire with the quality of copper being equal to that obtained at a plating current of only 100 amperes per square foot Without any lithium. Lithium was added in further amounts up to 0.4 ounce per gallon of the electrolyte, and the maximum plating current density was maintained at substantially 120 amperes per square foot.

Steel panels 4" x 6" x 0.085" having a surface roughness of approximately 12 microinches were placed in the electrolyte. A periodic reverse current having a plating portion of 60 seconds and a deplating portion of 45 seconds was applied to the panels. The current density during the plating portion of the cycle was 200 amperes per square foot, and 180 amperes per square foot during the deplating portion. In 56 minutes, copper of a thickness of 0.0011 inch was plated on the panels. The roughness of the plated copper was approximately 3 microinches. This constitutes a leveling of 75%.

There was then added to the electrolyte of this Example II lithium hydroxide to provide 0.04 ounce per gallon of lithium. It was found possible to use the same periodic reverse current time cycles as were previously employed but at a higher current density of 260 amperes for the plating portion and 240 amperes for the deplating portion. In 56 minutes, there was deposited 0.0015 inch thickness of copper on the panels. The roughness of the copper was 1 microinch. The leveling was approximately 92%. Thus, not only was the plating operation faster, but the quality of the copper was improved.

In another test, copper was plated for only 30 minutes from the electrolyte of this Example II with 0.04 ounce/gal. of lithium therein, until 0.0011 inch of copper was deposited. The copper plated had a roughness of 2 microinches-a leveling of 83%.

A standard copper cyanide electrolyte containing neither zinc nor thiocyanate was prepared and tested with various periodic reverse current cycles. Lithium was then added, and at a concentration of 0.05 ounce per gallon of lithium, the electrolyte enabled copper to be plated therefrom at a rate 20% higher than could be plated from the same electrolyte without the lithium. Furthermore, the copper plated from the electrolyte with lithium was brighter over a wider range of plating current densities than could be secured without the lithium being present.

Example III A bath of the following composition was prepared:

Ounces per gallon Copper 11.2 Free NaCN 1.1 NaOI-I 4.8 NazCOs 9.3 Zinc 0.15

Copper was plated from this bath on zinc die castings using a periodic reverse current having a 20 second plating period and an 8 second deplating period, the current density during the deplating period being 75% of the plating current density. The bath was operated at the maximum current density possible to produce good copper without burning. In twenty minutes 0.0008 inch of copper was plated on the die castings.

To the electrolyte of this Example III, 0.04 ounce per gallon of lithium was added. It was found that the bath could be operated at a 25% increase current density to produce copper plate of the same quality as was produced previously at the lower current density. In twenty minutes 0.001 inch of high quality copper was deposited on the zinc die castings.

It has been found that where copper electroof normal standards for copper.

deposits are required for the carrying of electrical current that the lithium alone enables such electrodeposits to be produced with a current-carrying capacity equal to better than Thus, wire has been plated with copper from copper cyanide electrolytes containing lithium as the sole addition agent and found to have a conductivity of well over of the standards for copper conductors.

It will be appreciated that the lithium must be replenished fromtime to time inasmuch as it is lost from the electrolyte by being plated out and by drag-out. Also, the zinc and thiocyanate, if they are present, must be similarly replenished.

The cyanide electrolytes of this invention are preferably agitated or stirred to move the electrolyte past the member being plated at the rate of at least 1 foot per second, and preferably at a rate of the order of 10 feet per second during the plating operation. Filtering of the electrolyte is recommended inasmuch as the electrodeposits produced are so smooth and bright that they will show the presence of any suspended matter in the electrolyte.

While emphasis has been made herein on the fact that lithium enables the plating of copper at a greater rate than possible without the lithium, it has been found that the plated copper is usually smoother and brighter, everything else being equal, than when plated from the same electrolyte without the lithium. With the addition of lithium, mirror-bright copper has been obtained over a wide range of current densities from numerous copper cyanide plating electrolytes. Furthermore, the presence of lithium enables an increase of leveling to be obtained compared to that possible without the lithium. It should be understood that while with periodic reverse current, substantial leveling is obtained, any increase in leveling is highly desirable. Lithium enables an additional reduction in roughness of plated copper. The operation of a copper cyanide electrolyte with lithium in conjunction with periodic reverse current enables the plater to approach the desired goal of 1 to 2 microinches of final roughness for a thickness of 0.001 inch of copper plated on a surface having from 10 to 15 microinches of roughness before plating.

The copper cyanide electrolytes with lithium alone or with zinc and thiocyanate, may be employed for direct-current plating. However, the benefits of lithium are not as pronounced with direct current as they are with periodic reverse current.

It is intended that all the matter contained in the above description shall be deemed to be illustrative and not limiting.

I claim as my invention:

1. An aqueous copper cyanide electroplating electrolyte comprising, in combination, from 2 to 15 ounces per gallon of copper, alkali metal cyanide in an amount to complex the copper in solution and to provide from 0.5 to 5 ounces per gallon of free alkali metal cyanide, from 2 to 12 ounces per gallon of alkali metal hydroxide, from 1 to 20 ounces per gallon of alkali metal carbonate, and from 0.01 to 0.4 ounce per gallon of lithium.

2. The electrolyte of claim 1 wherein from 0.004 to 0.57 ounce per gallon of zinc is present.

3. The electrolyte of claim 1 wherein from 0.1

to 10 ounces per gallon of thiocyanate are present.

4. The electrolyte of claim 1 ,wherein from 0.004 to 0.57 ounce per gallon of zinc and from 0.1 to 10 ounces per gallon of thiocyanate are present.

5. In the process of electroplating copper on a member, the steps comprising applying to the member as a cathode an aqueous cyanide electrolyte having dissolved therein from 2 to 15 ounces per gallon of copper, alkali metal cyanide in an amount to compex the copper in solution and to provide from 0.5 to 5 ounces per gallon of free alkali metal cyanide, from 2 to 12 ounces per gallon of alkali metal hydroxide, from 1 to 20 ounces per gallon of alkali metal carbonate, and from 0.01 to 0.4 ounce per gallon of lithium, passing a plating electrical current through the electrolyte for a period of time of from 1/5 second to 300 seconds, then passing a deplating electrical current through the electrolyte for a period of time to apply to the member from 10% to 90% of the coulombs applied during the preceding plating period, and continuing the alternate plating and deplating until a desired thickness of copper has been plated on the member.

6. The process of claim 5 wherein the electrolyte also contains from 0.004 to 0.57 ounce per gallon of zinc and from 0.1 to 10 ounces per gallon of thiocyanate.

7. In the process of electroplating copper on a member, the steps comprising immersing the member as a cathode in an aqueous cyanide electrolyte having dissolved therein from 2 to 15 ounces per gallon of copper, alkali metal cyanide in an amount to complex the copper in solution and to provide from 0.5 to 5 ounces per gallon of free alkali metal cyanide, from 2 to 12 ounces per gallon of alkali metal hydroxide, from 1 to 20 ounces per gallon of alkali metal carbonate, and from 0.01 to 0.4 ounce per gallon of lithium, and passing a plating electrical current through the electrolyte to plate copper on the member.

8. In the process of electroplating copper on a member, the steps comprising applying to the member as a cathode a cyanide copper electrolyte having dissolved therein from 0.01 to 0.4 ounce of lithium per gallon of the electrolyte and passing a plating electric current through the electrolyte to plate copper on the member.

9. An improved aqueous copper cyanide electroplating electrolyte comprising from 0.01 to 0.4 ounce of lithium per gallon of the electrolyte, the lithium being dissolved in the electrolyte.

References Cited in the file of this patent Meyer et al., Transactions Electrochemical Society, vol. 73 (1938), pp. 377-413.

Lloyd et al., Transactions Electrochemical Society, vol. 94 (1948), pp. 341-352. 

9. AN IMPROVED AQUEOUS COPPER CYANIDE ELECTROPLATING ELECTROLYTE COMPRISING FROM 0.01 TO 0.4 OUNCE OF LITHIUM PER GALLON OF THE ELECTROLYTE, THE LITHIUM BEING DISSOLVED IN THE ELECTROLYTE. 